Angular distribution of $J/\psi$ decays in dimuon channel in 800 GeV proton-copper collisions

TL;DR

This study measures the angular distribution and polarization of $J/$ decays in proton-copper collisions at 800 GeV, revealing a transition from transverse to longitudinal polarization with increasing $x_F$.

Contribution

It provides the first detailed polarization measurements of $J/$ in this energy regime, highlighting the $x_F$ dependence of polarization states.

Findings

$J/$ produced with slight transverse polarization at small $x_F$

Polarization shifts to longitudinal at $x_F$ > 0.6

No observed $p_T$ dependence of polarization parameter $$

Abstract

The anuglar distribution of $J/\psi$ decays in the $\mu^{+}\mu^{-}$ channel in 800 proton-copper collisions has been measured for $x_F$ > 0.25. The polarization parameter $\lambda$ is extracted in 1 GeV of $p_T$ and 0.1 of $x_F$ bins for two magnet configugrations with different acceptances. The data indicate that the $J/\psi$'s are produced with a slight transverse polarization at small $x_F$, which turns to longitudinal at $x_F$ > 0.6. No $p_T$ dependence of $\lambda$ is observed. Theoretical calculations are needed in order to interpret the measurements.